This invention relates to power inverters, and more particularly to an overload protection system for use with power inverters.
Power supply systems employing inverters require automatic overload protection circuits to safeguard not only the expensive inverter components, but also the environment of the load. Once the overload condition has been corrected, operation of the inverter should be restored automatically. This is particularly desirable in aerospace applications where a simple fuse for overload protection cannot be used because it is not convenient, or sometimes possible, to reset or replace the fuse. But even in applications where there is no inconvenience in resetting a fuse, such as in a solor energy system for home use or in an electric vehicle, it would be desirable to safeguard the power inverter by automatically shutting the inverter down during any overload condition that may be specified. Once the overload condition has been corrected, the operation of the inverter should be restored automatically. This not only satisfies safety requirements, but assures reliability of the inverter and its power supply such as solar cells or batteries. Moreover, this conserves power. Reliability and conservation of power are both very important in aerospace applications and civil system applications where the inverter is expected to resume operation once the overload condition has been cleared, and there is limited power available.
Several circuit arrangements have been devised to provide overload protection. One frequently used arrangement employs a current-limiting feedback loop. During normal operation, the power supplied to power switching transistors from the power source passes through a low resistance path consisting of a series transistor and resistor to the centertap of the primary of the inverter transformer. Initial current flows through another higher impedance path to allow operation of the inverter transistors. As the AC voltages in the inverter increase, a rectified voltage is developed capable of driving series transistor into saturation. Thus only the low resistance of the fully turned on series transistor and resistor limits current flow to the inverter transistors.
A high surge current resulting from an overload or short circuit increases the voltage drop across the series resistor and this biases a second transistor into conduction to supply current to the inverter through a much larger series resistor and to drive the first series transistor into cutoff. Current flow to the inverter transistors is thus throuugh a much higher resistance path, thereby limiting current to a safe value.
One disadvantage of this arrangement is that when the load current is at a nominal level, the first series transistor and resistor constantly dissipate power. This produces a considerable decrease in efficiency of the power inverter. Another disadvantage of the arrangement is that under some overload conditions, the developed back-bias is not sufficient to effect complete cutoff of first the series transistor. Stress applied to this first series transistor under such overload conditions may thus cause damage.
Another frequently used protection circuit is of the active overload protection type. An overload sampling circuit operates generally in a manner similar to a monostable multivibrator or one-shot. Under normal operating conditions the protection circuit remains in a rest state similar to that of the untriggered state of a one-shot. Current surges produced by overloading or shorting are monitored by a current sensor in series with the load. The rectified voltage produced therefrom is compared with a reference voltage to generate an error signal. Under appropriate conditions, the error signal controls the output of a one-shot multivibrator to inactivate the clock pulse generator for a period which is determined by the time constant of the one-shot. The state of the load is reexamined cyclically until the abnormal condition is cleared, following which inverter operation resumes.
One disadvantage of this arrangement is the tendency to produce system oscillation, which is accompanied by periodic generation of high current spikes. The spikes radiate intolerable noise to adjacent equipment susceptible to such noise. Moreover, the power switching devices of this system are exposed to very high stress conditions periodically because of high current spikes during overload which cause premature failures of components. Another disadvantage is that since the time constant of the overload protection circuit if fixed, normal operation is not resumed immediately on clearance of the abnormal condition. If a very short time constant is chosen to offset this limitation, the resulting frequency of oscillation is very high, radiating even more undesirable noise.